Heating devices with temperature self-regulating properties are used in the automotive industry. However, such heaters are employed where flexibility of the heater is not at issue. For example, such heaters are used on mirrors located outside of the vehicle. These heaters are printed upon a rigid biaxially oriented polyester film. See, e.g. U.S. Pat. Nos. 4,931,627 and 4,857,711, both assigned to the assignee of the present application.
Heaters for automotive vehicle seats that are currently available offer less than adequate performance due to several undesirable attributes. Current heaters are known to build up static electricity, which damages the heater controller circuit when it is discharged. Another shortcoming is that current seat heater design, in which the heater elements are copper wire and design creates several problems in that heating is localized to the area of the wires, creating an undesirable heating pattern where the areas in the vicinity of the wire are too hot and areas removed from the wire are too cool. Moreover, since the heating wire per se does not possess any means for regulating the temperature (that is, copper wire and the like is incapable of sensing that it has become too hot), a sophisticated temperature controller is required for regulating the temperature of the seat heater. This creates a challenging design problem for the engineer, which could be avoided if the heater construction per se was self-regulating and could increase or decrease the amount of heat produced as necessary.
Furthermore, when heating a seat in an automotive vehicle, it is evident that the seat heater construction must be flexible, durable, and able to withstand the demands of the operating environment, which include the potentially degradative effects of prolonged exposure to heat and the flow of electricity.
It would be desirable if a heater for an automotive seat were deigned so that a uniform amount of heat could be distributed over the area to be heated. Likewise it would be desirable if a seat heater could be designed in which, if desired, the amount of heat delivered to particular area could be varied as a design parameter, so that if it is deemed that certain areas should be warmer than others for a given design (or cooler, as the case may be), the heater could be constructed to accommodate this variation.
Furthermore, since the comfort of a vehicle seat is attributable to its flexibility, it would be desirable if the seat heater construction was flexible so that its presence in the seat complimented the other flexible components of the seat construction. It would be additionally desirable if the seat heater construction incorporated a flexible fabric layer. It would be highly advantageous if the heater components could be applied to the fabric using well known printing and coating techniques, which could be used to construct a heater quickly and easily, and relatively cheaply. Also, application techniques such as printing or coating could be used to make uniform or varying applications of component materials, which could provide for the uniform distribution of heat, or if desired, variations in the amount of heat.
Positive temperature coefficient (PTC) materials exhibit variable electrical resistance with temperature. As the temperature of the material increases, the electrical resistance also increases. The resistivity of the material increases so current flow is reduced, limiting heat flow. In essence, positive temperature coefficient compositions are used to form temperature self-regulating coatings. PTC materials are known in the art. Exemplary disclosures concerning these materials can be found in U.S. Pat. Nos. 5,206,482 and 5,151,747, among others.